It's a carnival night in Paris, but Châtelet police commissioner Nicolas Le Flock is not in the mood for a holiday, because the body of a young man was found in the city near the walls of the Fort-Léveque prison. At the first inspection, the prison director recognizes him as one of the prisoners who tried to escape. They even found a rope twisted from sheets, with which the fugitive tried to climb over the wall. But for an experienced policeman, the excessive “obviousness” of the crime is suspicious, and he finds another solution to the mystery of the murder, in which very high-ranking persons are involved, including the head of British intelligence, Lord Ashbury himself.